The present invention relates to a process for preparing removable sleeves to be photoengraved for use in printing presses.
The invention in particular relates to metal sleeves to be used in rotogravure and/or flexographic printing.
It is known that rotogravure printing takes advantage of a cylindrical printing form giving rise to the desired images, which form is obtained by a photoengraving process creating small cells inside which ink is retained to be then transferred to the paper.
The surface on which the cells are engraved following the different methods, i.e. the traditional, halftone, conversion, electronic and still other methods, is generally made of copper.
This surface is obtained by electroplating on iron cylinders and after photoengraving it is submitted to a protection chromium-plating process.
At each new series of images, the cylinders are processed in qualified engraver""s shops and then sent to the user for mounting on rotogravure printing presses.
Taking into account the important sizes and weight of each cylinder and at the same time the great fragility of the engraved surfaces or the surfaces to be engraved, clear problems exist for transportation and storage, which will bring about heavy costs and consequent locking up of capital.
In order to overcome the above drawbacks and reduce costs, use of cylindrical metal sleeves generally of nickel, to be removed from the cylinders, has been envisaged.
A copper coating is deposited on the surfaces of these sleeves, which is similar to the coating deposited on the surfaces of traditional cylinders, in order to enable a normal photoengraving.
To facilitate removal of these sleeves from the cylinders different methods have been adopted. The most famous method comprises a series of holes opening onto the surface of the cylinder and enabling creation of a compressed air cushion between the cylinder surface and the sleeve.
Taking into account the great difference in weight between the sleeves, 2-3 kilograms, and the cylinder, a 100-200 kilograms, the great advantages offered by the removable sleeves over the traditional cylinders appear obvious. Management costs are reduced due to the elimination of grinding and balancing operations, servicing, costs for staff and transportation. The print quality is improved without formation of grinding imprecisions or expensive waste.
However, taking into account the present situation on the market tending to a greater variability in wraps with a reduction in press-runs, where there is also a need for a further reduction in costs so as to increase competitiveness, elimination of the sleeves after a press-run has been completed is still-heavy and capable of adversely affecting the market.
Consequently, a process has been adopted in which a copper coating is formed on the sleeves made of nickel in such a manner that detachment of the film when the engraving is to be replaced is made easy.
According to a preferred method, a copper film is applied to the sleeves made of nickel together with a separation product, then formation of the copper coating to be photoengraved is carried out.
Then, on the copper-plated nickel sleeve, once the coating has been removed, a new coating for a new engraving is deposited by electroplating.
The separation product generally consists of a silver-based solution or of passivation agents such as chromates, dichromates or others.
Separation of this copper coating from the sleeve, when a new photoengraving is to be carried out, is obtained with the aid of mechanical means. Then, after application of the separation product, a new coating is deposited.
This process has important advantages as compared with a direct deposition of the coating to be photoengraved onto the cylinder.
However, cost of the sleeves is still very high due both to the high cost of nickel and to the long times required for formation of the different layers: the copper film, the separation product, the copper coating.
It is an object of the present invention to provide a process for making sleeves to be photoengraved for rotogravure and/or flexographic printing, capable of enabling production of the sleeves directly in a galvanic bath without use of any nickel sleeve on which copper is to be electroplated.
It is a further object of the invention to enable accomplishment of sleeves, preferably completely made of copper, which can be easily removed from the cylinder on which they are produced and at the same time can ensure an optimal observance of the required tolerances in particular for rotogravure printing.
A still further object of the invention is to greatly reduce costs for the removable sleeves, as regards both the material and the time required for forming them, which therefore will bring about important advantages both from an economical and from a practical point of view, as clarified in the following.
The foregoing and further objects that will become more apparent in the course of the present description are substantially achieved by a process for preparing removable metal sleeves to be photoengraved for printing machines as claimed in one or more of the accompanying claims.
A detailed description of a preferred but not exclusive method of preparing removable metal sleeves for rotogravure and/or flexographic printing is now given hereinafter.